Conventional optical communications systems employ direct modulation schemes where the current to be injected into a laser diode being a light source is directly modulated with the input signal so that an optical signal modulated with the input signal is outputted. A laser diode experiences a phenomenon (chirping) in which the oscillation wavelength of the laser diode changes due to the chirp characteristics. Therefore, where an optical signal outputted from a laser diode is transmitted over a long distance, the waveform of the optical signal deteriorate and the signal characteristics deteriorate due to the influence of the waveform dispersion in the optical fiber.
Discussions have been made on the use of the millimeter wave band (30-300 GHz), which can easily be reserved, in wireless communications systems in order to significantly increase the transmission rate. Where a coaxial cable is used as a transmission path for communications between base stations in a wireless communications system using the millimeter wave band, a signal of the millimeter wave band has a significant loss along the transmission path. Accordingly, in order to transmit a signal of the millimeter wave band via a coaxial cable, it is necessary to provide amplifiers at intervals on the order of 10 m, thus significantly increasing the cost. Therefore, for the millimeter wave band signal transmission, it is necessary to use an optical fiber with little transmission loss. However, the frequency response characteristic of a currently commercially available laser diode is about 10 GHz. Therefore, the laser diode cannot respond to a signal of a very high frequency such as those in the millimeter waveband. Thus, it is impossible to directly modulate a millimeter wave band signal with a laser diode.
In view of this, it has been proposed in the art to use an optical transmission device provided with a Mach-Zehnder type external optical modulator (hereinafter referred to as an “MZ type modulator”) using lithium niobate that has little chirping and is capable of responding to high-frequency signals, in a case where an optical signal is transmitted over a long distance or where a high-frequency signal of the millimeter wave band is transmitted as an optical signal.
FIG. 23 shows a general configuration of the MZ type modulator. An MZ type modulator 900 shown in FIG. 23 includes a crystalline substrate 901, a first optical waveguide 902, a second optical waveguide 903, an electrode 904, an electrode 905, a bias input terminal 906, and an RF (Radio Frequency) signal input terminal 907.
Light outputted from the light source is inputted to the MZ type modulator 900. The input light is branched into two beams for two waveguides, i.e., the first optical waveguide 902 and the second optical waveguide 903. A voltage is applied between the electrode 904 and the electrode 905 provided on the crystalline substrate according to the bias voltage inputted from the bias input terminal 906 and the voltage of the RF signal inputted from the RF signal input terminal 907. If an electric field occurs between the electrode 904 and the electrode 905, the refractive index of the first optical waveguide 902 changes. Light propagating through the first optical waveguide 902 changes its phase as the refractive index of the first optical waveguide 902 changes. Light propagating through the first optical waveguide 902 and that propagating through the second optical waveguide 903 are combined together, whereby the MZ type modulator 900 outputs a signal modulated with an RF signal.
An MZ type modulator experiences a phenomenon (DC drift) in which the relationship between the bias voltage and the optical output varies from the initial state depending on various conditions such as aging and temperature changes. FIG. 24 shows the DC drift occurring in an MZ type modulator. The solid line shown in FIG. 24 represents the initial phase of the MZ type modulator. The one-dot chain line shown in FIG. 24 represents the phase of the MZ type modulator, which has been varied due to the DC drift. As shown in FIG. 24, the signal characteristics of the MZ type modulator deteriorate because the relationship between the bias voltage and the output optical signal level varies due to the DC drift. Patent Document 1 discloses a technique for solving the problem caused by the DC drift of an MZ type modulator.
FIG. 25 shows a configuration of a conventional optical transmission device described in Patent Document 1. The optical transmission device shown in FIG. 25 includes a light source 911, a signal source 912, an amplifier 913, an RF input terminal 914, an MZ external optical modulator 915, a light branching section 916, alight receiving section 917, an amplifier 918, a differential amplifier 919, a reference voltage input terminal 920, and a bias voltage input terminal 921.
With the optical transmission device shown in FIG. 25, the optical carrier outputted from the light source 911 is inputted to the MZ external optical modulator 915. A signal of a predetermined frequency f1 outputted from the signal source 912 is amplified through the amplifier 913 and is inputted to the MZ external optical modulator 915. The MZ external optical modulator 915 outputs an optical signal obtained by modulating the optical carrier with the signal of the frequency f1. The light branching section 916 branches the optical signal into two signals one of which is outputted to the optical transmission path (not shown) and the other to the light receiving section 917 as a monitoring optical signal. The light receiving section 917 converts the monitoring optical signal to an electric signal. The differential amplifier 919 amplifies the difference between the electric signal, which has been amplified through the amplifier 918, and the reference voltage inputted from the bias voltage input terminal 920, and outputs the resulting signal as an error signal. The error signal is inputted to the bias voltage input terminal 921 of the MZ external optical modulator 915. Thus, the optical transmission device shown in FIG. 25 compares the monitoring optical signal with a predetermined reference voltage and controls the bias voltage to be applied to the MZ external optical modulator to thereby control the point of operation.
In recent years, discussions have been made on the use of a single-sideband suppressed-carrier MZ external optical modulator (hereinafter referred to as an “SSB-SC optical modulator”) including at least three MZ type modulators as shown in FIG. 23. FIG. 26 is a schematic diagram showing a configuration of an SSB-SC optical modulator. As shown in FIG. 26, an SSB-SC optical modulator 930 includes a first Mach-Zehnder interferometer (hereinafter referred to as an “MZ interferometer”) 931, a second MZ interferometer 932, and a third MZ interferometer 933.
The SSB-SC optical modulator shown in FIG. 26 branches an optical carrier inputted from a light source (not shown) into two. One of the branched optical carriers is inputted to the first MZ interferometer 931, and the other to the second MZ interferometer 932. The first MZ interferometer 931 modulates the optical carrier with the RF signal inputted from an RF signal input terminal 935 to output an optically-modulated signal. The second MZ interferometer 932 modulates the optical carrier with the RF signal inputted from an RF signal input terminal 937 to output an optically-modulated signal. The third MZ interferometer 933 adjusts the phase of the optically-modulated signal inputted from the first MZ interferometer 931 by using the bias voltage V3 inputted from a bias input terminal 939, and combines together the phase-adjusted optical signal and the optically-modulated signal inputted from the second MZ interferometer 932. As a result, a single-sideband suppressed-carrier optically-modulated signal is outputted from the SSB-SC optical modulator.
The principle of operation of the SSB-SC optical modulator will now be discussed using arithmetic expressions. The photoelectric field E1 of the optically-modulated signal outputted from the first MZ interferometer 931 is represented by Expression 1 below.
                                                                                          E                  1                                =                                ⁢                                                      cos                    ⁢                                          {                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  m                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                +                                                                              ϕ                                                          DC                              ⁢                                                                                                                          ⁢                              1                                                                                ⁡                                                      (                                                          V                              1                                                        )                                                                                              }                                                        +                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                -                                                  m                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                                    )                                                                                                                                              =                                ⁢                                                                            cos                      ⁡                                              (                                                                                                            ω                              0                                                        ⁢                            t                                                    +                                                                                    ϕ                                                              DC                                ⁢                                                                                                                                  ⁢                                1                                                                                      ⁡                                                          (                                                              V                                1                                                            )                                                                                                      )                                                              ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                              -                                      sin                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              ϕ                                                          DC                              ⁢                                                                                                                          ⁢                              1                                                                                ⁡                                                      (                                                          V                              1                                                        )                                                                                              )                                                                                                                                                              ⁢                                                      sin                    ⁡                                          (                                              m                        ⁢                                                                                                  ⁢                        cos                        ⁢                                                                                                  ⁢                                                  ω                          1                                                ⁢                        t                                            )                                                        +                                      cos                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                    ⁢                                                                                  ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                              +                                      sin                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                    ⁢                                                                                  ⁢                                          sin                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                                                                                                                    =                                ⁢                                                                                                    J                        0                                            ⁡                                              (                        m                        )                                                              ⁢                                          cos                      ⁡                                              (                                                                                                            ω                              0                                                        ⁢                            t                                                    +                                                                                    ϕ                                                              DC                                ⁢                                                                                                                                  ⁢                                1                                                                                      ⁡                                                          (                                                              V                                1                                                            )                                                                                                      )                                                                              -                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                        m                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                    ⁢                                                                                  ⁢                    sin                                                                                                                                          ⁢                                                      (                                                                                            ω                          0                                                ⁢                        t                                            +                                                                        ϕ                                                      DC                            ⁢                                                                                                                  ⁢                            1                                                                          ⁡                                                  (                                                      V                            1                                                    )                                                                                      )                                    +                                                                                    J                        0                                            ⁡                                              (                        m                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                                    +                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                        m                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                                                                                                                          =                                ⁢                                                                                                    J                        0                                            ⁡                                              (                        m                        )                                                              ⁢                                          {                                                                        cos                          ⁡                                                      (                                                                                                                            ω                                  0                                                                ⁢                                t                                                            +                                                                                                ϕ                                                                      DC                                    ⁢                                                                                                                                                  ⁢                                    1                                                                                                  ⁡                                                                  (                                                                      V                                    1                                                                    )                                                                                                                      )                                                                          +                                                  cos                          ⁢                                                                                                          ⁢                                                      ω                            0                                                    ⁢                          t                                                                    }                                                        -                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                        m                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                                                                                                                                          ⁢                                  {                                                            sin                      ⁡                                              (                                                                                                            ω                              0                                                        ⁢                            t                                                    +                                                                                    ϕ                                                              DC                                ⁢                                                                                                                                  ⁢                                1                                                                                      ⁡                                                          (                                                              V                                1                                                            )                                                                                                      )                                                              -                                          sin                      ⁢                                                                                          ⁢                                              ω                        0                                            ⁢                      t                                                        }                                                                                                        =                                ⁢                                                                            J                      0                                        ⁡                                          (                      m                      )                                                        ⁢                                      {                                                                  cos                        ⁡                                                  (                                                                                                                    ω                                0                                                            ⁢                              t                                                        +                                                                                          ϕ                                                                  DC                                  ⁢                                                                                                                                          ⁢                                  1                                                                                            ⁡                                                              (                                                                  V                                  1                                                                )                                                                                                              )                                                                    +                                              cos                        ⁢                                                                                                  ⁢                                                  ω                          0                                                ⁢                        t                                                              }                                                                                      ⁢                                  -                                            J              1                        ⁡                          (              m              )                                ⁢                      {                                          sin                ⁡                                  (                                                                                    (                                                                              ω                            0                                                    +                                                      ω                            1                                                                          )                                            ⁢                      t                                        +                                                                  ϕ                                                  DC                          ⁢                                                                                                          ⁢                          1                                                                    ⁡                                              (                                                  V                          1                                                )                                                                              )                                            +                              sin                ⁡                                  (                                                                                    (                                                                              ω                            0                                                    -                                                      ω                            1                                                                          )                                            ⁢                      t                                        +                                                                  ϕ                                                  DC                          ⁢                                                                                                          ⁢                          1                                                                    ⁡                                              (                                                  V                          1                                                )                                                                              )                                            -                                                sin                  ⁡                                      (                                                                  ω                        0                                            +                                              ω                        1                                                              )                                                  ⁢                t                            -                                                sin                  ⁡                                      (                                                                  ω                        0                                            -                                              ω                        1                                                              )                                                  ⁢                t                                      }                                              Exp        .                                  ⁢        1            
In Expression 1, m·cos(ω1t) is the RF signal inputted from the RF signal input terminal 935. In the expression, φDC1 (V1) is the phase difference between two light waves passing through the optical waveguides of the first MZ interferometer 931, which is caused by the bias voltage V1 inputted from a bias input terminal 934. In the expression, m is the degree of phase modulation, ω0 is the angular frequency of the optical carrier to be inputted, ω1 is the angular frequency of the RF signal. In the expression, Jn is the nth-order Bessel function. In the following description, the second-order and subsequent components of the Bessel function are ignored.
The second MZ interferometer 932 receives m·sin(ω1t), which is obtained through a Hilbert transformation of the RF signal m·cos(ω1t) inputted to the first MZ interferometer 931 from the RF signal input terminal 937. The photoelectric field E2 of the optical signal outputted from the second MZ interferometer 932 is represented by Expression 2 below.
                                                                                                              E                    2                                    ⁡                                      (                    t                    )                                                  =                                ⁢                                                      cos                    ⁢                                          {                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  m                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                +                                                                              ϕ                                                          DC                              ⁢                                                                                                                          ⁢                              2                                                                                ⁡                                                      (                                                          V                              2                                                        )                                                                                              }                                                        +                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                -                                                  m                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                                    )                                                                                                                                              =                                ⁢                                                                            cos                      ⁡                                              (                                                                                                            ω                              0                                                        ⁢                            t                                                    +                                                                                    ϕ                                                              DC                                ⁢                                                                                                                                  ⁢                                2                                                                                      ⁡                                                          (                                                              V                                2                                                            )                                                                                                      )                                                              ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                              -                                      sin                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              ϕ                                                          DC                              ⁢                                                                                                                          ⁢                              2                                                                                ⁡                                                      (                                                          V                              2                                                        )                                                                                              )                                                                                                                                                              ⁢                                                      sin                    ⁡                                          (                                              m                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                                                  ω                          1                                                ⁢                        t                                            )                                                        +                                      cos                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                    ⁢                                                                                  ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                              +                                      sin                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                    ⁢                                                                                  ⁢                                          sin                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                )                                                                                                                                                                    =                                ⁢                                                                                                    J                        0                                            ⁡                                              (                        m                        )                                                              ⁢                                          cos                      ⁡                                              (                                                                                                            ω                              0                                                        ⁢                            t                                                    +                                                                                    ϕ                                                              DC                                ⁢                                                                                                                                  ⁢                                2                                                                                      ⁡                                                          (                                                              V                                2                                                            )                                                                                                      )                                                                              -                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                        m                        )                                                              ⁢                    sin                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                    ⁢                                                                                  ⁢                    sin                                                                                                                                          ⁢                                                      (                                                                                            ω                          0                                                ⁢                        t                                            +                                                                        ϕ                                                      DC                            ⁢                                                                                                                  ⁢                            2                                                                          ⁡                                                  (                                                      V                            2                                                    )                                                                                      )                                    +                                                                                    J                        0                                            ⁡                                              (                        m                        )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                                    +                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                        m                        )                                                              ⁢                    c                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          ω                      0                                        ⁢                    t                                                                                                                          =                                ⁢                                                                            J                      0                                        ⁡                                          (                      m                      )                                                        ⁢                                      {                                                                  cos                        ⁡                                                  (                                                                                                                    ω                                0                                                            ⁢                              t                                                        +                                                                                          ϕ                                                                  DC                                  ⁢                                                                                                                                          ⁢                                  2                                                                                            ⁡                                                              (                                                                  V                                  2                                                                )                                                                                                              )                                                                    +                                              cos                        ⁢                                                                                                  ⁢                                                  ω                          0                                                ⁢                        t                                                              }                                                                                      ⁢                                  -                                            J              1                        ⁡                          (              m              )                                ⁢                      {                                          sin                ⁡                                  (                                                                                    (                                                                              ω                            0                                                    +                                                      ω                            1                                                                          )                                            ⁢                      t                                        +                                                                  ϕ                                                  DC                          ⁢                                                                                                          ⁢                          2                                                                    ⁡                                              (                                                  V                          2                                                )                                                                              )                                            +                              cos                ⁡                                  (                                                                                    (                                                                              ω                            0                                                    -                                                      ω                            1                                                                          )                                            ⁢                      t                                        +                                                                  ϕ                                                  DC                          ⁢                                                                                                          ⁢                          2                                                                    ⁡                                              (                                                  V                          2                                                )                                                                              )                                            +                                                cos                  ⁡                                      (                                                                  ω                        0                                            +                                              ω                        1                                                              )                                                  ⁢                t                            -                                                cos                  ⁡                                      (                                                                  ω                        0                                            -                                              ω                        1                                                              )                                                  ⁢                t                                      }                                              Exp        .                                  ⁢        2            
In Expression 2, φDC2 (V2) is the phase difference between two light waves passing through the optical waveguides of the second MZ interferometer 932, which is caused by the bias voltage V2 inputted from a bias input terminal 936. In Expressions 1 and 2, J0(m) represents the optical carrier component, and the J1(m) component represents a sideband component. Based on Expressions 1 and 2, the bias voltage that best suppresses the optical carrier component J0(φ) is the point where the value φDC1 (V1) shown in Expression 1 and the value φDC2 (V2) shown in Expression 2 are both π.
When the values φDC1 (V1) and φDC2 (V2) are both π in Expressions 1 and 2, the photoelectric field E3 of the optically-modulated signal outputted from the third MZ interferometer 933 is represented by Expression 3 below.
                                                                        E                3                            =                            ⁢                              2                ⁢                                                                  ⁢                                                      J                    l                                    ⁡                                      (                    m                    )                                                  ⁢                                  {                                                                                    sin                        ⁡                                                  (                                                                                    ω                              o                                                        +                                                          ω                              l                                                        +                                                                                          φ                                                                  DC                                  ⁢                                                                                                                                          ⁢                                  3                                                                                            ⁡                                                              (                                                                  V                                  3                                                                )                                                                                                              )                                                                    ⁢                      t                                        +                                                                                                                                                          ⁢                                                      sin                    ⁡                                          (                                                                        ω                          o                                                -                                                  ω                          l                                                +                                                                              φ                                                          DC                              ⁢                                                                                                                          ⁢                              3                                                                                ⁡                                                      (                                                          V                              3                                                        )                                                                                              )                                                        ⁢                  t                                }                            -                              2                ⁢                                                      J                    I                                    ⁡                                      (                    m                    )                                                  ⁢                                  {                                                                                    cos                        ⁡                                                  (                                                                                    ω                              o                                                        +                                                          ω                              l                                                                                )                                                                    ⁢                      t                                        -                                                                                                                                        ⁢                                                cos                  ⁡                                      (                                                                  ω                        o                                            -                                              ω                        l                                                              )                                                  ⁢                t                            }                                                                          =                            ⁢                                                2                  ⁢                                                            J                      l                                        ⁡                                          (                      m                      )                                                        ⁢                                      {                                                                                            sin                          ⁡                                                      (                                                                                          ω                                o                                                            +                                                              ω                                l                                                            +                                                                                                φ                                                                      DC                                    ⁢                                                                                                                                                  ⁢                                    3                                                                                                  ⁡                                                                  (                                                                      V                                    3                                                                    )                                                                                                                      )                                                                          ⁢                        t                                            -                                                                        cos                          ⁡                                                      (                                                                                          ω                                o                                                            +                                                              ω                                l                                                                                      )                                                                          ⁢                        t                                                              }                                                  +                                                                                                      ⁢                              2                ⁢                                                      J                    l                                    ⁡                                      (                    m                    )                                                  ⁢                                  {                                                                                    sin                        ⁡                                                  (                                                                                    ω                              o                                                        -                                                          ω                              l                                                        +                                                                                          φ                                                                  DC                                  ⁢                                                                                                                                          ⁢                                  3                                                                                            ⁡                                                              (                                                                  V                                  3                                                                )                                                                                                              )                                                                    ⁢                      T                                        +                                                                  cos                        ⁡                                                  (                                                                                    ω                              o                                                        -                                                          ω                              l                                                                                )                                                                    ⁢                      t                                                        }                                                                                        Exp        .                                  ⁢        3            
In Expression 3, φDC3(V3) is the phase difference between two optically-modulated signals inputted to the third MZ interferometer 933 caused by the bias voltage V3 inputted from a bias input terminal 938. As shown in Expression 3, it can be seen that the bias voltage with which the optically-modulated signal outputted from the third MZ interferometer 933 is a single-sideband signal with suppressed optical carrier component is the point where φDC3(V3) is π/2. Thus, with the SSB-SC optical modulator shown in FIG. 26, it is possible to obtain a suppressed-carrier single-sideband optically-modulated signal by operating each MZ interferometer at a predetermined point of operation. FIG. 27 shows a schematic diagram of the spectrum of the optically-modulated signal outputted from the SSB-SC optical modulator.
Patent Document 2 discloses an SSB-SC optical modulator bias controlling method and device for controlling the bias voltage supplied to each MZ interferometer based on a monitoring low-frequency signal superimposed on an optically-modulated signal so as to suppress the DC drift occurring in each MZ interferometer. FIG. 28 shows a configuration of the conventional SSB-SC optical modulator bias controlling device described in Patent Document 2. Referring to FIG. 28, the conventional SSB-SC optical modulator bias controlling device includes the first MZ interferometer 931, the second MZ interferometer 932, the third MZ interferometer 933, a first bias controlling section A, a second bias controlling section B, a third bias controlling section C, and a photodetector 959. The first bias controlling section A controls the bias voltage supplied to the first MZ interferometer 931 and generates a monitoring low-frequency signal fa so that the low-frequency signal fa is superimposed on the optically-modulated signal outputted by the first MZ interferometer 931. The second bias controlling section B controls the bias voltage supplied to the second MZ interferometer 932 and generates a monitoring low-frequency signal fb so that the low-frequency signal fb is superimposed on the optically-modulated signal outputted by the second MZ interferometer 932. The third bias controlling section C controls the bias voltage supplied to the third MZ interferometer 933 and generates a monitoring low-frequency signal fc so that the low-frequency signal fc is superimposed on the optically-modulated signal outputted by the third MZ interferometer 933. The photodetector 959 detects the optically-modulated signal outputted from the SSB-SC optical modulator.
The first bias controlling section A controls the bias voltage supplied to the first MZ interferometer 931 based on the low-frequency signal fa included in the output signal from the photodetector 959. The second bias controlling section B controls the bias voltage supplied to the second MZ interferometer 932 based on the low-frequency signal fb included in the output signal from the photodetector 959. The third bias controlling section C controls the bias voltage supplied to the third MZ interferometer 933 based on the low-frequency signal fc included in the output signal from the photodetector 959. Thus, the first to third bias controlling sections each control the bias voltage supplied to the corresponding MZ interferometer based on the monitoring low-frequency signal superimposed on the optically-modulated signal, thereby suppressing the DC drift occurring in the MZ interferometer.    Patent Document 1: Japanese Laid-Open Patent Publication No. 6-67128 (page 5, FIG. 1)    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-318052    Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-133824